This application is related to Japanese applications No. HEI 11(1999)-111084 filed on Apr. 19, 1999 and No. HEI 11(1999)-248984 filed on Sep. 2, 1999, whose priorities are claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an oscillator and to a method for adjusting the oscillation characteristics of the oscillator. More specifically, the present invention relates to a small oscillator used in mobile telephones, mobile data terminals, wireless LAN transmitter/receivers, satellite communications terminals, GPS receivers and other types of wireless communication devices operating at a high frequency band, and to a method for adjusting the oscillation characteristics of the oscillator. An oscillator according to this invention is well suited to reducing the size of a module that is a major component of the oscillator used particularly in high frequency applications operating at hundreds of megahertz and higher.
2. Description of Related Art
As terminal devices for high frequency communication systems, such as mobile telephones, have gotten smaller, so have oscillation circuit modules, such as voltage control oscillators (VCO), which are one kind of high frequency components used in such terminal devices.
The characteristics of individual components and variations in wiring pattern dimensions in substrates cannot be ignored in such small oscillation modules for high frequency applications, particularly at frequencies of hundreds of megahertz and higher. It is therefore necessary in practice to adjust each individual module so that the oscillation frequency, for example, is within a specific design range.
One method used for this adjustment as taught in Japanese Unexamined Patent Application (kokai) 6-13807 is to adjust an inductor part of an oscillation circuit by changing the length and/or width of a conductor pattern. This is accomplished by cutting a part of a circuit pattern formed on a component mounting surface of a circuit board by mechanical means, such as sandblasting, or optical means, such as a laser. Other methods seeking to achieve an even smaller module by means of a multilayer circuit board provide a part of an inductor (see Japanese Patent 2662748) or capacitor (see Japanese Patent 2531000) of the oscillation circuit in circuit board layers.
FIGS. 17 and 18 show an exemplary oscillator in which an inductor part of an oscillator is drawn to a circuit board surface. As shown in, FIG. 17, a back ground conductor 22, an internal ground conductor 23 and an inductor conductor 24 of a strip line between the conductors 22 and 23 are formed within a printed circuit board 21, forming a tri-plate structure. A part of the strip line inductor conductor 24a is connected through a through hole 25 to a conductor pad 24b mounted on the substrate surface. The electrical length of the inductor can then be changed by appropriately trimming this surface conductor pad 24b on the substrate surface as shown at notch 26 in FIG. 18, thereby adjusting inductance to vary oscillator characteristics such as oscillation frequency.
Another adjustment method is illustrated in FIG. 19. As shown in this section view of an oscillator, a capacitor part of the oscillator is formed inside a substrate, and one electrode of the capacitor electrode disposed on the substrate surface is trimmed to adjust capacitance as a means of adjusting oscillator characteristics. In this example, an inductor part of a resonance circuit is incorporated as a strip line 24 of the tri-plate structure inside a printed circuit board 21 as in the above example, and one end of the inductor is exposed via a through hole 25 on to the surface 27 as a surface electrode 28. That is, a capacitor parallel-connected to the inductor and forming a resonance circuit is mounted within the circuit board with the surface electrode 28 opposed to the internal ground conductor 23 with intervention of the dielectric circuit board. The surface electrode 28 is trimmed to adjust the electrode surface area, and thereby adjust capacitance, that is, adjust the oscillator.
In the above-described prior-art examples, while part of the circuit is formed inside the circuit board so as to reduce device dimensions, both the methods also expose part of the internal conductor pattern on the component mounting surface of the circuit board so that the exposed part (part 24b in FIG. 17, and part 28 in FIG. 19) is trimmed, to adjust oscillator module characteristics. This method of internalizing part of the circuit as a means of reducing size is therefore the same as methods in which parts are not internalized in that a trimming pad occupies a certain amount of area on the component mounting surface.
A metal shield cap covering the component mounting surface is provided for most such modules to protect the mounted components and prevent electromagnetic interference with neighboring parts. This shield cap can be mounted either before or after adjusting the oscillator.
When the shield cap is installed after trimming a circuit component on the component mounting surface of the circuit board to adjust the oscillator, the adjustment must be accomplished to anticipate any shift in characteristics resulting from the later addition of the shield cap. If the shield cap is installed before the adjustment, a laser can be used for trimming through a trimming slit or hole provided in the shield cap, and the trimming slit or hole is then sealed with a conductive sealant.
In the former method, trimming must allow for any shift in the oscillator frequency or other module characteristics resulting from the shield cap. As noted above, however, because variations in characteristics of individual modules are great, the adjustment is not possible with a uniform offset, and the precise adjustment is not possible with this method.
In the latter method, dust and debris from the trimmed part become trapped inside the shield cap because trimming is accomplished after the shield cap is mounted. Such dust and debris can easily adhere to surrounding parts, resulting in a possible loss of reliability.
One possible method of resolving this problem is to draw an internal conductor to the back side of a circuit board for trimming. In this case, however, a trimming pad must be provided, and this occupies some area on the back of the circuit board. This makes it difficult to shield the back of a module. A shield cap such as provided on the front surface of the circuit board must therefore be provided. This method is therefore not practical for applications seeking to downsize the module.
Another oscillator adjustment method proposing a solution to this problem is taught in Japanese Unexamined Patent Application (kokai) 9-153737. This method accomplishes laser trimming perpendicular from the back of a circuit board to the inside to adjust electrode area of a multilayer internal capacitor.
FIG. 20 is a section view of a module in this method. In this module a capacitor 27 of a resonance circuit is internalized in a circuit board by stacking electrodes 27a and 27b with a dielectric layer of the circuit board therebetween. As indicated by an arrow 28 in FIG. 21, laser trimming perpendicular to and from the back of the module adjusts the area of the electrodes 27a and 27b of the internal capacitor 27, and thus adjusts the module.
While this method solves the surface area problem described above, an electrode structure of a capacitor must be a multilayer structure comprising at least three or four layers in order to achieve a sufficient capacitance adjustment range. The result is a multilayer circuit board, increasing circuit board thickness and cost.
Furthermore, YAG lasers are typically used for trimming because they can produce a necessary power and can gather light to a spot of several ten micrometers or more. On the other hand, the fundamental wavelength of a YAG laser is 1.06 xcexcm. As shown in FIG. 22, reflectivity at this wavelength is extremely high with copper and other metals used for conductors to be trimmed, necessitating even more power for trimming. The result is that a circuit board is easily overtrimmed to the back side of the conductor being trimmed, thus lowering insulation properties as a result of partial carbonization, and making it difficult to achieve a desired trimming effect. Furthermore, when the conductor imbedded in a dielectric must be trimmed together with the dielectric as in the above-noted method, it is difficult to appropriately adjust the laser power to achieve the desired trimming.
With consideration for the above problems, an object of the present invention is therefore to provide a compact, high performance, high reliability oscillator (module) having a structure which allows an internal conductor pattern constituting primarily a resonance circuit inductor to be trimmed directly from the back of a circuit board. A further object of the invention is to provide a high density adjustment method for said oscillator (module).
To achieve the above objects, the present invention provides an oscillator which comprises: a circuit board having a dielectric layer therein; an oscillator circuit comprising its components and an inductor element, the components being mounted on the front surface of the circuit board and at least a part of the inductor element being internally disposed in the dielectric layer; and a back conductor covering a major part of the back surface of the circuit board; wherein the back conductor has a slit or pinhole which allows the dielectric layer and the internally disposed part of the inductor element to be partially cut by a laser beam passing through the slit or pinhole for adjustment of an oscillator characteristic.
In other words, the present invention provides the slit or pinhole in the back conductor covering the major part of the back surface of the circuit board. A part of the internal inductor element is then cut out (trimmed) by emitting a laser beam through this slit or pinhole to burn out a part of the inductor element (to form a conductive pattern) together with the dielectric layer intervening between the back conductor and said inductor element. A desired oscillator characteristic is thus achieved.